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Tankless water heaters are terrible....

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When I inquiried, a few plumbers and my regular A/C technician told me that they see a lot of problems with tankless water heaters - maintenance-wise. They do not recommend them for that reason and because their cost vs. a regular water heater or heat pump doesn't make sense. But again, every situation is different, so yours may prove that a tankless is the way to go.

My motivation for starting this thread is that people often choose tankless because they want a more efficient water heater. The improvement over a regular water heater is a <10% reduction in energy use vs >70% reduction with a heat pump.

In addition to the higher maintenance costs there's a loss of demand response potential. The ability to control demand is going to get more critical as wind and solar generation expand. Few people would be willing to time their showers around wind generation but with a large tanked heater you sync its heat cycle to when surplus wind or solar is available without any inconvenience. The worst thing for the grid would be a consistent spike every evening and morning when people with tankless electric heaters take showers....
 
My motivation for starting this thread is that people often choose tankless because they want a more efficient water heater. The improvement over a regular water heater is a <10% reduction in energy use vs >70% reduction with a heat pump.
This may be situational based. I see significantly more savings for a gas tankless over a gas tank heater. More like 50%.

But the initial post was on electric tankless. I agree minimal improvement. It’s easy to fully insulate an electric heater.

I don’t really buy the maintenance concern. My tankless has lasted much longer than tank heater. Only problem is a gecko/lizard that got stuck in the fan causing it to sound super loud. Easy removal.
 
This may be situational based. I see significantly more savings for a gas tankless over a gas tank heater. More like 50%.

But the initial post was on electric tankless. I agree minimal improvement. It’s easy to fully insulate an electric heater.

I don’t really buy the maintenance concern. My tankless has lasted much longer than tank heater. Only problem is a gecko/lizard that got stuck in the fan causing it to sound super loud. Easy removal.

The benefit to using a heat pump tanked heater is the 'heat pump' part not the 'tank' part. Resistance vs Resistance tankless is slightly better.

In areas that have hard water tankless heaters build scale more rapidly. This is another benefit to heat pump water heaters. The refrigerant coils surround the tank and don't get hot enough to form scale.
 
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My motivation for starting this thread is that people often choose tankless because they want a more efficient water heater. The improvement over a regular water heater is a <10% reduction in energy use vs >70% reduction with a heat pump.

In addition to the higher maintenance costs there's a loss of demand response potential. The ability to control demand is going to get more critical as wind and solar generation expand. Few people would be willing to time their showers around wind generation but with a large tanked heater you sync its heat cycle to when surplus wind or solar is available without any inconvenience. The worst thing for the grid would be a consistent spike every evening and morning when people with tankless electric heaters take showers....
Scheduling heating also lets a person optimize the heat pump operation during the warmest times of the day.

To a very large degree, DHW production should be:
During excess PV
COP > 4
50% heat recycling

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As for the purported efficiency of on-demand,
Just demand from the construction folks that pipes be insulated.
And save yourself costs related to high load use.
 
I have had a Takagi gas tankless water heater for over 5 years and it has worked well and reduced my NG use. I can't provide any hard numbers.

In the future I plan to replace it with either a heat pump or an electric tankless but haven't done much research yet. This thread is interesting.
Keep in mind that natural gas tank-less heaters (and I assume hybrid) require a double wall vent. If you are retrofitting that can be a significant expense.

Also, if you use an electric heat pump in below freezing climates the calculations of energy use when they switch over to electric element will be vastly different.
 
Oh my, this could be worse than all the birds and bats killed by wind turbines! BTW, what sort of loud noise does a gecko/lizard make when it is stuck in a fan?
It's a squirrel cage blower fan, so it put it off balance. Sounded like a jet taking off.

A gas tankless beats a gas tank heater any day of the week in my book. I've saved more than enough to pay for it 3x over in the 10 years it has been in operation (replacing a 10 year old propane tank heater that leaked). It even paid for the $10 pump I use to circulate vinegar through it once a year.

Yes, I'd prefer heat pump tank powered by electric solar - will get there some day. Have some house upgrades I need to get done before I go the solar route. Thanks for the point on the scale build up. It makes sense!
 
Keep in mind that natural gas tank-less heaters (and I assume hybrid) require a double wall vent. If you are retrofitting that can be a significant expense.

Yes, that was a significant expense for my tankless. Still paid off for me. I do think a large reason I had significant savings is the fact that my propane tank was in the attic, exposed to the cold in the winter.

Hybrid works with standard B-Vent.

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I looked into heat pump hot water heaters. In the winter they suck heat out of the room they are installed in. In my case it would be the laundry/mud room. So in winter the room with the door open the most would be the heat source. It would transfer the heat from the central heating system into the water cooling the room. So the BTUs from the propane furnace will be captured and transferred to the water, doesn't seem like free heat like in the summer. At least 1/2 the year it would be better to burn the propane in a highly efficient gas water heater.
 
I looked into heat pump hot water heaters. In the winter they suck heat out of the room they are installed in. In my case it would be the laundry/mud room. So in winter the room with the door open the most would be the heat source. It would transfer the heat from the central heating system into the water cooling the room. So the BTUs from the propane furnace will be captured and transferred to the water, doesn't seem like free heat like in the summer. At least 1/2 the year it would be better to burn the propane in a highly efficient gas water heater.

What part of WA are you in? My sister had propane heat. She lives near Tacoma. I helped her upgrad recently to heat pumps for the house and water heater.

The BTU cost of propane and electricity are almost identical... 1g of propane costs ~$2.50 and with a standard furnace will provide ~23kWh of heat. If sucking heat out of the house in the winter is a concern you can set the HPWH to use resistance heat in the winter...

The Rheem also has fittings for ducts... so you could draw and exhaust the air from outside. If it's too cold to function it automatically shifts to resistance.
 
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The BTU cost of propane and electricity are almost identical...

Is that statement referring to resistance heating or to a heat pump?

If sucking heat out of the house in the winter is a concern you can set the HPWH to use resistance heat in the winter...

If everyone designs everything in a place like New England to just switch to resistance heating when the temperature outside is -5F, does that mean we're going to have to build massive grid capacity to cope with the rare very cold days when the heat pumps can't function well, with most of that grid capacity going unused 95%+ of the time?
 
We have three Rinnai gas tankless heaters here at work and there is always something wrong with at least one of them. Currently two out of the three are down and all three are less than five years old. They need a lot of maintenance to keep hard water deposits from building up in the heat exchanger, which is something that my facilities people don't seem interested in doing as often as the heaters would like.

My conventional Peerless hot water boilers (low pressure, natural gas) are still going strong after 27 years. Too bad they're only 80% efficiency.
 
Keep in mind that natural gas tank-less heaters (and I assume hybrid) require a double wall vent. If you are retrofitting that can be a significant expense.
This is not universally true. I have a Navien condensing tankless gas water heater that uses all PVC ducting. If a tankless gas water heater requires a double wall vent it is throwing away a LOT of heat.
 
Is that statement referring to resistance heating or to a heat pump?

Resistance.

If everyone designs everything in a place like New England to just switch to resistance heating when the temperature outside is -5F, does that mean we're going to have to build massive grid capacity to cope with the rare very cold days when the heat pumps can't function well, with most of that grid capacity going unused 95%+ of the time?

No. Peak heating load at night would be hard pressed to exceed peak cooling load in the summer. There are also heat pumps that are capable of operating below -5F in addition to ground source heat pumps. This heat pump keeps operating down to -20F. If resistance heat becomes more popular then electricity rates go up and advanced heat pumps look more appealing. The market provides ;)
 
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Peak heating load at night would be hard pressed to exceed peak cooling load in the summer.

Even if everyone switches to using only electric heating in the winter?

When you're doing calculations with R value of insulation, isn't it the case that the absolute value of the temperate difference between inside and outside is proportional to the number of BTU/h that leaks through the insulation, and thus if you're trying to keep a building heated to 72F when the outside temperature is -8F you have an 80F difference, whereas if the outside temperature is 99F and you're trying to cool off the building to 72F, you only have to deal with a 27F difference, which means you need roughly 3 times as much BTU/h capacity in the winter than in the summer? And then the differences get further magnified by the heat pump being more efficient and higher capacity at 99F than -8F? And then you also get to deal with preheating air going into the Energy Recovery Ventilator when the outside temperature is -8F? And then convection might cause more of a tendency for heat to leak out the vent pipe for the drain in the winter than to leak in during the summer?

I think the only reason summer loads are currently higher than winter loads is that air conditioning [edit: almost] always uses electricity, but heating usually doesn't in current practice.

If resistance heat becomes more popular then electricity rates go up and advanced heat pumps look more appealing. The market provides ;)

If we end up with proper real time rate metering, yes. But in the pre solar panel era, folks without air conditioning were probably subsidizing those who did have air conditioning under the fixed cost per residential kilowatt hour billing scheme.
 
I think the only reason summer loads are currently higher than winter loads is that air conditioning [edit: almost] always uses electricity, but heating usually doesn't in current practice.

If the objective is to reduce fossil fuel consumption heat pumps are the best way to accomplish that. If electric loading during cold snaps becomes an issue there are renewable ways to provide heat sustainably like wood stoves. The new EPA rated stoves are remarkably efficient.

We can electrify a good portion of our winter loads before we exceed the surplus capacity we have for summer cooling. And the way things are headed we'll need less heat in the winter and more cooling in the summer anyway :(
 
While wood burning during cold snaps ought to end up being carbon neutral, I'm not sure it's otherwise ideal from a local air quality perspective.

And I also forgot to point out that solar panels tend to have less output in the winter than the summer, if we're going to rely on solar plus batteries (where the batteries are not really sized to shift energy by more than about 15 hours) for a substantial fraction of our power.

But if we end up wanting to remove carbon from the atmosphere through a process powered by renewable electricity, maybe we can suspend that carbon removal process during cold snaps to balance the grid.